Lasting memories require synaptic modifications as well as communication between the synapse and the nucleus where activity-dependent gene expression is initiated. Elucidating the cellular mechanisms that allow active synapses to rapidly communicate to the nucleus is an outstanding challenge for neurobiology. Current models of synaptic-nuclear communication focus on the movement of second messengers or proteins between these distant cellular compartments or the propagation of action potential or ER evoked calcium waves into the nucleus. While translocation of molecules from the synapse to the nucleus clearly occurs, it is relatively slow and cannot account for the rapidity of experimentally observed nuclear responses to synaptic activity. Moreover, synaptic activity has been demonstrated to rapidly trigger nuclear events independent of action potential generation and when the ER is depleted of calcium. This proposal will test the hypothesis that synaptic activation can depolarize the ER membrane generating an electrical signal that propagates throughout the cell to the nucleus. The ER and nuclear membranes are polarized, have a high membrane resistance, and contain voltage and calcium gated ion channels - biophysical features that support the generation and propagation of electrical signals. ER-mediated electrical signaling would have privileged access to voltage gated channels in the nuclear envelop initiating or facilitating nuclear calcium influx. This idea will be tested by targeting genetically-encoded fluorescent voltage sensors to the ER membrane in order to image real time changes in ER membrane potential in response to synaptic activation. Individual or small clusters of synapses that contain ER will be activated with two-photon glutamate uncaging allowing the precise stimulation of synapses that are in closest proximity to the ER. Complimentarily, channel rhodopsin will be targeted to the ER membrane to determine if ER membrane depolarization is sufficient to trigger nuclear calcium events. If successful, the results of this study will redefine the biological role of the ER, establish a new mode of intercellular cellular communication, resolve a longstanding question in neurobiology, and develop imaging tools that are broadly useful to the neuro- and cell biology communities.